The invention relates to a plasma nozzle, in particular for pretreating surfaces, the nozzle comprising a tubular casing forming a nozzle channel through which a working gas is passed, an electrode disposed coaxially in the nozzle channel, and a counter electrode surrounding the nozzle channel.
A plasma nozzle of this type is disclosed in DE 195 32 412 A corresponding to U.S. Pat. No. 5,837,958 and serves, for example, for pretreating the surfaces of plastic (synthetic resin) materials such that coating of the surface with adhesive, printing inks and the like is made possible or facilitated. Such a pretreatment is necessary because plastic surfaces can normally not be wetted with liquids and do therefore not accept the printing ink or the adhesive. The pretreatment modifies the surface structure of the plastic material in such a manner that the surface can be wetted with liquids having a relatively large surface tension. The surface tension of the liquids with which surface can just be wetted is an indicator for the quality of the pretreatment.
The known plasma nozzle provides a relatively cool but nevertheless highly reactive plasma jet which has approximately the shape and dimensions of a candle flame and therefore permits also the pretreatment of profiled workpieces having a relatively deep relief. Thanks to the high reactivity of the plasma jet, a very short pretreatment time is sufficient, so that the workpiece can be moved past the plasma jet with a relatively high velocity. The relatively low temperature of the plasma jet therefore permits also the pretreatment of heat sensitive plastic materials. Since no counter electrode on the rear side of the workpiece is necessary, the surfaces of arbitrarily thick block-like workpieces, hollow bodies and the like can be pretreated without difficulties. For an even treatment of larger surface areas, the cited publication purposes an array of a plurality of staggered plasma nozzles. This, however, requires complex installations.
For pretreatment of larger surface areas, DE 298 05 999 U discloses an apparatus in which two plasma nozzles are mounted eccentrically and with parallel axes on a common rotating head, so that, when the surface is scanned with the rotary head, pretreatment is achieved in a stripe which has a width corresponding to the diameter of the rotating head. This apparatus is however not suitable for treating bulged surfaces the radius of curvature of which is in the order of the diameter of the rotating head. Moreover, the eccentric arrangement of at least two nozzles and the relatively high rotary speed lead to the occurrence of forces of inertia and gyroscopic forces when the rotating head is moved along more than one axis, for example with the aid of a robot arm.
In general, the known plasma the nozzles eject the plasma in axial direction of the nozzle channel. In case of workpieces having a complicated shape, this has the drawback that the locations to be treated are sometimes difficult to reach, in particular, when the nozzle is moved along the workpiece by means of a robot.